Aging is inevitably associated with diminished regeneration capacity of stem cells. We choose the skeletal muscle as a model system to study mechanisms that regulate the influence from aged environment on stem cell function. Aged skeletal muscle has reduced levels of FGF2 and elevated Wnts and TGF2, leading to decreased proliferation of resident stem cells (so called satellite cells) and increased fibrosis during regeneration. The bioavailability of FGF2, Wnts and TGF2 is regulated by sulfated heparan sulfate. This proposal investigates heparan sulfate-dependent mechanisms that regulate the transmission of age-related environmental signals to satellite cells during skeletal muscle regeneration. The first set of experiments focuses on roles of two extracellular heparan sulfate 6-O-endosulfatases (Sulfs) in differential regulation of the bioavailability of age-related signals. Sulfs enzymatically remodel heparan sulfate 6-O-sulfation, thereby Sulfs reduce Wnts and TGF2 binding to heparan sulfate, while disrupting FGF2 interaction with the receptor. Therefore, Sulfs are hypothesized to promote age-augmented Wnt and TGF2 signaling, while repressing age-reduced FGF2 signaling, leading to impaired function of satellite cells. This hypothesis will be tested by comparing the efficiency of myogenesis, fibrosis and age-related regeneration signaling in aged control, systemic and satellite cell-specific Sulf double mutant mice using a combination of in vivo regeneration and in vitro culture assays. The second set of experiments will test whether heparin, which is similar to heparan sulfate of Sulf-deficient mice in the structure and signaling function, will improve the efficiency of skeletal muscle regeneration in an aged environment. The results of these investigations are expected to lead to the discovery of Sulf- and heparan sulfate-dependent mechanisms that regulate signal communication between satellite cells and the aged muscle environment. Such knowledge may open new venues for prevention and therapy of impaired muscle regeneration by age.